Automatically controlled instrument systems having carriages which move in two coordinate directions over a work surface of a table are well-known. Typically, a first carriage traverses the table in one coordinate direction parallel to the work surface and a second carriage mounted on the first carriage moves relative to the first carriage in the other coordinate direction. When an instrument is mounted on the second carriage, composite movement of both carriages allow the instrument to be translated to any point over the region of the work surface traversed by the carriages. Accurate positioning of the carriages and, consequently, of the instrument, is achieved by numerical controls which may operate either from an on-line data generator or from previously programmed data. Such a numerical control system is described in U.S. Pat. No. 3,887,903 issued to Martell on June 3, 1975 and entitled "Interactive Man-Machine Method and System for Grading Pattern Pieces and for Producing an Apparel Marker" assigned to the assignee of the present invention.
Sheet material, such as fabric and plastics to be cut or paper on which graphical data is plotted, is placed on the work surface of the table associated with automatically controlled instrument systems which perform cutting and plotting functions. In such systems it is necessary to hold the sheet material in place before cutting or plotting begins in addition to holding cut pieces from the sheet material after the cutting instrument has passed over a portion of the sheet material.
Previously developed hold-down techniques have utilized a vacuum which is applied to a table associated with an automatically controlled instrument system. Such systems are illustrated in U.S. Pat. No. 3,180,608 issued to Fischer on Apr. 27, 1965 and entitled "Vacuum Holding of Thin Pliable Material"; U.S. Pat. No. 3,765,289 issued to Gerber et al on Oct. 16, 1973 and entitled "Vacuum Hold-Down Apparatus"; and U.S. Pat. No. 3,815,221 issued to Pearl on June 11, 1974 and entitled "Method for Holding Sheet Material by a Vacuum HoldDown". Additionally, previously developed vacuum hold-down techniques have utilized an air impermeable sheet placed over the sheet material to be cut in order to maintain the sheet material in a fixed position such that when a vacuum is applied, the vacuum produces forces against the air impervious material to compress and hold the sheet material before cutting. Such systems are illustrated in U.S. Pat. No. 3,598,006 issued to Gerber et al. on Aug. 10, 1971 and entitled "Method for Working on Sheet Material and Other Objects" and U.S. Pat. No. 3,742,802 issued to Maerz on July 3, 1973 and entitled "Sheet Material Cutting Apparatus Including a Vacuum HoldDown System Having a Roller Mechanism for Handling Air-Impermeable Sheets". Additionally, mechanical hold-down techniques have been utilized, as illustrated in U.S. Pat. No. 3,841,187 issued to Gerber et al on Oct. 15, 1974 and entitled "Method and Apparatus for Holding Sheet Material".
Although vacuum hold-down techniques, in combination with mechanical techniques, have been generally utilized for maintaining sheet material in a fixed position on a work table, such previously developed systems have not provided uniform vacuum pressure over the surface of a work table nor a vacuum of sufficient strength to maintain the work piece and cut pieces fixed without employing an additional hold-down sheet over the sheet material being worked upon. Additionally, such previously developed systems have suffered from vacuum loss after sheet material is cut which results in a reduction in the holding and compacting efficiency of the vacuum hold-down system. One suggested technique of preventing a reduction in vacuum loss is illustrated in U.S. Pat. No. 3,682,750 issued to Gerber on Aug. 8, 1972 and entitled "Cutting Apparatus With Vacuum Hold-Down and Cut Sealing Means" in which tape is applied to cuts to join cut pieces together, thereby maintaining a continuous sheet of material over the work surface to maintain vacuum pressure.
Previously developed hold-down systems have also required that the entire work surface be covered with sheet material in order for sufficient vacuum pressure to be generated in order to maintain the sheet material in position. Such systems do not have the capability of maintaining pieces of sheet material of a size less than the entire work surface of a table in a fixed position.
An additional problem associated with previously developed vacuum hold-down systems is the degree of flatness of the work surface of the table. In automatically controlled instrument systems, it is essential that the work surface be flat to insure proper operation of the carriages and operating instruments. Such tables must be immune to warpage and contour changes due to humidity conditions.
A need has thus arisen for a vacuum hold-down system and table for use in an automatically controlled instrument system in which vacuum pressure, without the need for mechanical aids, is generated of sufficient strength to maintain sheet material in a fixed position on a work surface. Such a system must be capable of holding pieces which have been cut from a large piece of sheet material in addition to holding sheet material which is smaller in size than the entire area of the work surface. Additionally, such a system must include a table that is not susceptible to warpage due to humidity changes in the environment to insure a flat work surface. In such a vacuum hold-down system, a need has further arisen for a vacuum system that creates a high, more evenly distributed flow rate through the table with low vacuum pressure. Additionally, a need has arisen for a table for use with an automatically controlled instrument system that is lightweight and economical to manufacture and maintain. Further, a need has arisen for a method of manufacturing a table that has a flat work surface.